1. Field of the Invention
The present invention relates generally to a process of welding metals, and more particularly to a process of cold welding metals using ion beams to prepare the surfaces of metals to be joined.
2. Description of the Prior Art
The fabrication of large structures in space will require reliable and efficient methods of joining metal structural members together. At the present time, aluminum is considered the prime construction material because of its cost, strength-to-weight relationship, and ease-of-fabrication properties. Welded joints are being considered. However, most conventional methods of welding involve a substantial amount of electric power for the formation of a molten metal zone and subsequent wetting of the metal interfaces to be joined, which adversely affects the mechanical properties. Thus the strength in the heat affected weld area is less than that of the unaffected parent metal. Although additional heat treatment after welding to restore some of the degraded mechanical properties is possible, it is undesirable because of time, energy, and equipment necessary.
Aluminum can also be joined by a process called cold welding. It is well known that if two perfectly clean (i.e., no oxide layer) and flat metal surfaces are brought into intimate contact (.ltorsim.0.25 nm) with the atoms on these surfaces in registry, a solid state weld can occur. The normal interatomic forces of attraction which hold together the atoms of a single piece of metal will also hold the atoms in the bond made between the two separate pieces. Such bonds, in theory, will be as strong as those in the parent metal. To produce such a bond, referred to as cold welding, ideal mechanical and metallurgical conditions are required.
A typical metal surface on a microscopic scale consists of both large and small asperities which are generally covered with a film of oxides, nitrides, absorbed gasses, water vapor, organic and other contaminations. Before these surfaces can be cold welded, it is necessary to eliminate the contamination layer on the surfaces which prevents metal-to-metal bonding and to prevent recontamination of the clean surfaces prior to the welding operation. In addition, the mating surfaces are brought together under sufficiently high mechanical force to bring the atoms at the interface into intimate contact over a large portion of the area to be welded. Such pressure results in plastic flow at the high point of the asperities on the mating surface.
In conventional practice it is possible to achieve reliable cold welding in aluminum, as well as some other materials, for many applications by applying sufficient pressure to cause plastic flow of the material at the interface of the pieces to be welded. By using welding dies or indentors to cause plastic flow, the contamination oxide layers, which are normally more brittle than the parent metal, at the interface are broken up and dispersed allowing clean underlying metal to squeeze through to the interface for clean metal-to-metal bonding. Additionally, under plastic flow the atomic contact and registry between the two metals are greatly increased. Thus a good solid state weld is formed. Tests show that the best conventional cold welds in aluminum assemblies occur at about 60% deformation.
Such plastic flow and deformation, however, has several drawbacks. First, the temper of the metal is increased by cold working in the bond area so that it is less ductile than the parent metal and, second, the strength of the bond will eventually equal the strength of the thinned metal caused by the deformation.
Experimenters have investigated cold welding phenomena in clean vacuum environments largely to support studies of friction, wear, and lubrication of spacecraft materials. Various methods to obtain clean surfaces were employed, such as fracturing the material, wire brushing the surfaces, and etching with DC and RF plasmas. Thereafter these surfaces were forced together with sufficient loading pressure to bring surface asperities into intimate contact. The data shows that the resultant bond is strongly influenced by the specific nature and amount of contamination. As little as one or two monolayers of metallic oxides effectively interrupt or prevent metallic bonding.
Physically or chemically absorbed layers, such as oxides, are present on even the cleanest metal surfaces in normal atmosphere and a surface that is cleaned at one atmosphere (.about.760 torr) pressure will be recontaminated by a monolayer in less than 10.sup.-8 seconds. However, in a vacuum pressure of 10.sup.-6 torr, the recontamination time is on the order of a few seconds and the time to recontaminate progressively increases as the vacuum pressure decreases. Unfortunately, the tenacity with which oxide monolayers as well as other highly active materials are attached to most metal surfaces make them stable at vacuum levels well below their bulk vapor pressure. Therefore, an effective cleaning method must be employed to completely remove the contamination layer even in good vacuum environments.